The present invention generally relates to a direct conversion photon counting detector, an imaging system and a method to detect photons with a direct conversion photon counting detector.
Direct conversion photon counting detectors are well-known in the art for single photon detection in for instance astrophysics or medical imaging, such as computed tomography (CT), positron emission tomography (PET), single photon emission computed tomography (SPECT) and the like. A direct conversion photon counting detector is disclosed in J. D. Eskin et. al, Journal of Applied Physics, volume 85, Number 2, page 647-659, 15 Jan. 1999.
A bulk of a direct conversion photon counting detector is composed of a layer of a direct conversion material, usually a single-crystal semi-conductor. Inbound radiation (such as x-rays, γ-rays) is absorbed by the direct conversion material and, due to a photo-electric interaction, mobile electron-hole pairs are generated in the direct conversion material. Because the direct conversion material is placed between a detector cathode on one side and a detector anode on an opposite side, generated electrons move along an imposed electrical field towards the detector anode, while generated holes move in opposite direction towards the detector cathode. Approaching electrons induce a local charge in the detector cathode. The detector anode may be sub-divided into a series or a grid of electrode pixels. When each of the electrode pixels has a sufficiently small area and is individually read out, a value representing count of electrons that impacted each electrode pixel, based on the induced charge per pixel, can be determined.
The smaller the electrode pixel area, the narrower a detectable pulse width and the higher a detectable photon flux rate. However, there are limitations to reducing electrode pixel area, most importantly because of cross-talk between adjacent electrode pixels due to charge sharing. To obtain narrower pulses for still relatively large pixel areas, individual electrode pixels are usually sub-segmented, wherein only an as small as possible part of the electrode pixel area is dedicated to electron or hole collecting. This area is commonly named a collecting electrode. The collecting electrode is connected to a signal amplifier and a value for an amount of inbound electrons or holes collected by the collecting electrode is generated from an amplified signal. A remaining electrode pixel area is dedicated to direct inbound electrons or holes towards the collecting electrode. This is usually achieved by applying a higher potential difference between the collecting electrode and the remaining electrode pixel area (e.g. the collecting electrode is at ground potential, while the remaining electrode pixel area has a negative potential). The detectable pulse width is strongly reduced compared to a configuration where a non-sub-segmented pixel area is used for signal generation.
When the remaining electrode pixel area is only dedicated to steering the electrons or holes towards the collecting electrode, it is commonly referred to in the art as a steering electrode. An alternate configuration is a so-called coplanar grid. In this configuration also a current pulse is induced for an amount of inbound electrons or holes that still approach near the remaining electrode pixel area, which, similar to a steering electrode, is operated under a repelling electric potential such that electrons or holes are steered towards a collecting electrode. The repelling electrode in this case is named a non-collecting electrode in the art. In contrast to a configuration with a steering electrode, both the collecting and the non-collecting electrode are connected to signal amplifiers. Amplified signals from the collecting and non-collecting electrodes are subtracted from each other, optionally in a weighted fashion. Approaching inbound electrons or holes are initially far away from the targeted electrode pixel and an equal amount of signal is capacitively induced into the collecting and non-collecting electrodes. As a result from the signal subtraction the signals cancel each other, except when inbound electrons approach near the collecting electrode. At that moment the induced signals become significantly different from each other and a relatively short pulse width may be generated as a result of subtracting the signals, provided that the electrons or holes are collected by the collecting electrode only.
While both steering electrode and coplanar grid configurations manage to reduce pulse width, there is a need to further reduce the pulse width. It is inherent to these configurations that charges are trapped near the collecting electrode which polarizes the detector, causing the electrical field to become distorted. This effect is especially limiting performance of the steering electrode configuration. The effect is less severe for coplanar grids, since charges are usually distributed over larger areas in this configuration. However, there needs to be a relatively large potential difference (at least several tens of volts) between the closely neighboring collecting and non-collecting electrodes to ensure good operation, coplanar grids are technically quite challenging to design, manufacture and operate and compromises must be made to overcome the potential difference between the collecting and non-collecting electrodes while subtracting the signals.